Chicago police work the scene of a multiple shooting in the 4200 block of South Wells Street in Chicago in November 2012. (Chris Sweda, Chicago Tribune)

Chicago police did not report about a quarter of the aggravated assault and aggravated battery victims in its crime statistics in 2012, an audit by the city's top watchdog found.

The department failed to follow state guidelines by counting each aggravated assault or battery as one incident, not each victim as it should have, leading to the underreporting because of all the incidents that involved multiple victims, according to the inspector general's office.

The department said it has reported aggravated assaults and aggravated batteries in this way for many years, meaning Chicago's statistics on these crimes — including nonfatal shootings — have long gone underreported to the Illinois State Police and FBI.

The Tribune first highlighted how the department reported the number of shooting incidents, not victims, in a front-page story in 2010 that focused on one month during that summer.

Then, last July, the newspaper analyzed virtually every shooting in the first six months of 2013 and found that there had been more than 1,000 victims, far more than the department's own numbers since it reported only shooting incidents.

The 24-page IG report, which found no evidence suggesting any of the department's crime-reporting errors were intentional, took a look at a sampling of 383 assault-related crimes and found that the 72 aggravated assault and aggravated battery incidents had a combined 95 victims. That meant that by reporting only the number of incidents, Chicago police had failed to include 23 victims, thus underreporting 24 percent of the aggravated assault and aggravated batteries in that sampling.

The department of police Superintendent Garry McCarthy blamed the administration of his predecessor, Jody Weis, for not changing the way it tracks aggravated assaults and aggravated batteries.

In 2010, when Weis ran the department, state police issued a clarification to police agencies that aggravated batteries and aggravated assaults should be reported by the number of victims, not number of incidents. But the Weis administration failed to change its tracking approach, the department said. The issue was first raised to the McCarthy administration late last year by the IG's office, the department said.

“Upon learning of the reporting issue, CPD immediately launched an in-depth review of every single aggravated assault and aggravated battery that occurred during 2012 and 2013, to correct the tracking of these crimes and bring the city into stricter adherence with reporting standards,” the department said in a response printed in the IG report. “This in-depth review is ongoing, and where errors in victim-level reporting exist, they will be corrected.”

Weis, reached by phone Monday evening, said he didn't recall hearing of any such directive by state police in 2010.

John Eterno, a former New York City police captain who now researches how police departments keep track of crime data, said he found the IG audit superficial but said it pointed to what could be even more widespread crime reporting errors. Eterno, who read the report, suggested that Chicago's other crime categories be reviewed more thoroughly.

“Based on this report, a larger audit … would be called for,” said Eterno, a criminal justice professor at Molloy College in Rockville Centre, N.Y. “This is the tip of the iceberg.”

Indeed, in a statement, Inspector General Joseph Ferguson cautioned the audit did not look at the accuracy of crime reporting in “the field” — by officers in the street.

The report said the IG's office decided against auditing homicide statistics after determining the odds were low that police officials would downgrade homicides to involuntary manslaughter or reckless homicide. The IG cited “the heightened public scrutiny” on homicides in part for that decision.

The IG office noted that the accuracy of the crime statistics is critical for public confidence as well as the department's heavy reliance on them for its CompStat program, a data-driven strategy championed by McCarthy that uses statistics, street intelligence and weekly performance meetings to hold district commanders responsible.

A Chicago police spokesman said Monday night that despite the misreporting, police are deployed appropriately across the city.

The report also mentioned how other big-city police departments that follow CompStat or similar models have been scrutinized publicly about alleged underreporting of crime, particularly violent crime.

Criminologists explained that the CompStat model puts pressure on police commanders to keep crime statistics down in their districts if they want to further their careers and avoid being embarrassed by bosses in front of their peers at the sometimes-confrontational weekly meetings.